Ink jet printing inks are required to have improved fastness properties, such as highlighter resistance and scratch resistance, after the ink jet printing inks are applied to recording media. In order to satisfy this requirement, it is known that resin particles are added to ink to improve the fastness properties. The addition of resin particles can improve the binding between a coloring material and a recording medium or between coloring materials, thereby improving the fastness properties.
As an ink containing resin particles, Japanese Patent Laid-Open No. 2004-204079 describes an ink that contains at least one type of resin particles incapable of forming a film at ambient temperature and at least one type of resin particles capable of forming a film at ambient temperature to improve the scratch resistance of recording images. As the effects of including at least two types of resin particles, for example, the addition of resin particles having different glass transition temperatures may reduce the temperature dependence of the physical properties (such as storage modulus) of the resin particles between the glass transition temperatures after being fixed to a recording medium. When resin particles are added to ink to improve fastness, the resin particles must be fused to form a continuous film. However, if the environmental temperature is lower than the film-forming temperature of the resin particles, it is difficult to fuse the resin particles. In such a case, it is known that the addition of resin particles having a low minimum film-forming temperature can improve the film-forming ability.
However, in an ink described in PTL 1, the addition of resin particles to the ink may result in insufficient dispersion stability of the ink. Furthermore, an ink described in PTL 1 sometimes results in unstable ejection in an ink jet printing method (a thermal ink jet printing method) in which ink is ejected from a print head by the action of thermal energy. This is probably because the addition of resin particles increases the viscosity of the ink or because heat generated by the application of pulses to the ink produces a deposit on a thin-film resistor. Thus, stable ejection of ink requires the prevention of an increase in viscosity caused by the addition of resin particles, foaming of ink into a desired volume on a thin-film resistor, and repeated foaming and defoaming of ink at desired time intervals.